Multigrade lubricants are preferred because of their ability to operate under broad temperature ranges. High molecular weight polymers have been used in multigrade lubricants to maintain oil viscosity as equipment operating temperatures increase. A problem with the use of high molecular weight polymers is their shear stability. Shear stability describes the polymer's ability to maintain oil viscosity after exposure to shearing conditions. Shear stability is a measure of the loss of polymer's ability to provide thickening to a fluid. This loss is typically referred to as permanent shear loss (PSL). One way of measuring permanent shear loss is the Taper Bearing Shear Test (DIN 350-06). The viscosity of a lubricant is measured before and after the test and the percentage of shear loss is reported. Today's lubricating compositions are exposed to high shear conditions, such as gear driveline applications. A need exists for multigrade shear stable lubricants, such as gear lubricants.
Low molecular weight polymers, e.g. {overscore (M)}w less than 50,000, may be used to prepare multigrade lubricants. Often high treat rates for the low molecular weight polymers are required to obtain appropriate viscosity. A disadvantage of these polymers is their effect on low temperature viscometrics as measured in the Brookfield viscometer. Multigrade lubricants must have acceptable low temperature properties, e.g. acceptable viscosity at low temperatures.
Multigrade lubricants have been made from synthetic, e.g., polyalphaolefin fluids, and natural base fluids. However, the cost of the synthetic fluids is very high compared to mineral oils. With mineral oils (e.g. up to SAE 250N), it is difficult to obtain good viscometrics, i.e. kinematic viscosity and/or Brookfield viscosity. More specifically, the amount of polymer needed to thicken the oil at high temperatures causes undesirable low temperature viscosity.
Lubricating compositions serve to remove heat from operating equipment and to reduce metal-metal contact which lead to wearing. Today many pieces of equipment are reduced in size, which in turn has led to higher operating temperatures for the equipment. These higher temperatures, along with exposure to oxidizing media, such as air or water, may lead to increased oxidation of the lubricating composition. Today the drain intervals for lubricants have increased. When a lubricant has to operate for longer periods at higher temperatures, the lubricant is prone to viscosity increase. The viscosity increase is believed to be caused by polymerization of oxidized components of the lubricants. This increased viscosity renders the lubricant unfit for use. It is therefore desirable to have lubricants with improved oxidation resistance. More particularly, it is desirable to have lubricants which will withstand long periods of operation at high temperatures.
It is desirable to have ingredients which can form multigrade lubricants, especially gear, transmission and differential lubricants with good low and high temperature viscometrics and acceptable shear stability. A need exists for components which can provide the desired viscometrics for mineral oil based lubricants while also being shear stable and have good low temperature performance. Additionally a need exist for lubricants which can meet the above requirements and which provide improved oxidation resistance.